1. Technical Field
The present invention relates generally to semiconductor processing tools, and more particularly, to a feature dimension deviation correction system, method and program product.
2. Related Art
The use of feedback controllers in semiconductor processing has long been established in the fabrication of semiconductor integrated circuits by semiconductor manufacturing facilities. Until recently, wafers were treated as a batch or a lot and the same processing was performed on each of the wafers in the lot. The size of the lot varies depending on the manufacturing practices of the facility, but is typically limited to a maximum of twenty-five wafers. Measurements are routinely made on a few wafers in the lot after processing and adjustments made to the next lot to be processed based on these sample measurements. This method of control based on sample measurements on the lot and process recipe adjustments for the following lots is called lot-to-lot control (L2L). The process models and information necessary to modify the process recipes for L2L control are stored, and the computations are performed at the facility level.
Recently manufacturers of semiconductor processing equipment (SPE) have included the ability to measure each wafer immediately before and after the processing is performed. In particular, processing chambers are provided with integrated metrology tools such as those that implement scatterometry. Hence, the capability to measure each wafer on the same tool used for processing is called integrated metrology (IM). IM, in turn, allows measurement and feedforward or feedback adjustments at the wafer-to-wafer (W2W) level, or after the lot completes (i.e., L2L), or some variation in updating between W2W and L2L control. Conventional approaches, however, suffer from a number of drawbacks. First, current IM tools are typically optically-based, e.g., scatterometry-based, tools can be sensitive to deviations in underlying film thicknesses, which can change over time, between and within lots, and even across a wafer.
Second, conventional approaches assume any deviation from a target dimension is based on the process or processing tool that generates the feature. That is, conventional approaches assume measurements and measurement calibrations are correct, and do not determine the origin of any deviation of a feature's dimension from a target dimension. As a result, adjustments are typically applied only to the process tool that generated the feature, and upstream errors that are not detected by the pre-process IM tools are overlooked. In addition, calibration traditionally has been applied to external standard reference metrology measurement equipment, not IM tools. This type of calibration is static and does not compensate for drifts in IM tools, or in deviations of optical, physical or electrical properties between wafers and lots that may occur during processing and influence the IM measurement. For instance, U.S. Pat. No. 6,625,497 to Fairbarn et al. provides a processing module with integrated feedback and feed-forward metrology, but which uses a static measurement calibration, performed once prior to usage.
In reality, it is important to separate process equipment deviations, wafer state properties deviations and metrology deviations so that the correct adjustment can be made. Deviations in incoming wafer state can come from product differences, material changes, previous process equipment deviations in processing, and across wafer uniformity changes. Changes in wafer state can directly impact the result of a static process, or the process itself can vary. Metrology deviations might come from hardware replacement or differences between metrology tools. Current methods of feed forward and feedback L2L and W2W control make adjustments for processing deviations only, without separating the process equipment deviations, wafer state properties deviations and the metrology deviations.
Conventional methods have also not adjusted and improved to accommodate the advent of new optical IM techniques. Critical dimension (CD) scanning electron microscopy (SEM) measurements, primarily sensitive to the surface of a substrate, are a fundamentally different type of measurement method from the optical integrated metrology (IM), which can sense underlying materials as well. Although it is difficult to use the slow CDSEM metrology measurement for wafer-to-wafer control, usage of these tools may allow valuable information to be used by the IM to improve the accuracy and confidence of the measurements, even while the CDSEM measurements are only taken on a sample bases.
In addition to IM, processing tools used in the semiconductor industry, in general, have become complex processing systems including a number of process modules and integrated control systems. However, efficient usage of these features for tailored control and optimization of segments of the process sequence on a lot-to-lot basis and on a wafer-to-wafer basis has not been fully achieved. In particular, because of the high volume of data collected and short period of time between the measurements and subsequent processing of the wafer, it is necessary to provide the ability to perform wafer-to-wafer (W2W) control at the tool rather than at the facility level. Yet, this ability has not been implemented. For example, as noted above, feedback corrections are generally applied on a L2L basis with wafers within the lot all receiving the same correction. Since upstream process variations can be caused by the lack of tool-to-tool or module-to-module matching, reticle differences, chemical batch changes and simple process drifts, the feedback correction of incoming lots from a defined thread of tools is often set to equal to an Exponentially Weighted Moving Average (EWMA) of the lot deviation. Unfortunately, it can be difficult to track and control these threads of tools. W2W measurements, an EWMA-based feedback is no longer optimum. Another shortcoming of current IM processing tools is that they have not been integrated with facility processing systems, which control multiple processing tools, measurement tools and control systems within the overall facility.
In view of the foregoing, there is a need in the art for a way to address the problems of the related art.